1. Field of the Invention
The present invention relates to a composite metal molding including metal particles which welded mutually and resin carbide intervening among the metal particles, a method for manufacturing the composite metal molding and an electromagnetic driving element having a yoke forming a magnetic circuit upon energization to a field coil.
2. Description of the Related Art
There is known a powder molding process in which metal particles are compression molded in a mold with a resin used as a bonding material. A molding made by the powder molding process has the advantage of being very close in dimensions and shape to the mold and basically not requiring any post-work. Accordingly, the powder molding process can be said to be a process which is effective for manufacturing mainly a product of an expensive material or a product which is difficult to make by cutting work. The molding made by the powder molding process is limited in mechanical strength because of its structure including a resin as a bonding material among metal particles. Accordingly, the molding made by the powder molding process is often used as an element for which mechanical strength is not very important. For example, a molded magnetic member made by employing a rare earth magnet powder as metal particles is adopted as e.g. a circular columnar rotor in a motor. The scope of its application is further expanded to include e.g. the yoke or stator of a motor, the yoke or transformer of an actuator in an optical instrument and the core of a magnetic head which are all made by employing a soft magnetic material as metal particles.
In order to improve the magnetic properties of a magnetic member molded by employing a hard magnetic material as metal particles, it is effective to apply as high a molding pressure as possible to bring the metal particles close together to realize a high magnetic flux density. It is also effective to heat a compression molding to harden the resin among metal particles and then return the resin to ambient temperature to make it undergo thermal contraction, as disclosed in Japanese Patent Application Laid-open No. 7-176416 (1995). This makes it possible to bring the particles closer together and raise their coercive force owing to their thermal contraction strain to achieve an improved maximum energy product.
There is also known a product made by heating a compression molding extremely to weld metal particles together, while removing a bonding material completely in a degreasing step as according to a powder metallurgical sintering process, and thereafter sizing it to finish it into a desired size and shape. However, a molded magnetic body made by employing metal particles which are greatly affected by stress strain, such as a soft magnetic material, has its soft magnetic properties lowered by processing strain resulting from post-processing, such as sizing. It has also been likely that a molded magnetic body having an asymmetric complicated shape may not permit any such sizing. In order to overcome these inconveniences, Japanese Patent Application Laid-open No. 6-017103 (1994) proposes a method in which a sintered product is manufactured accurately by inserting a correcting member in a hollow compact and placing the compact on a plate having a protruding or recessed mark to support it in its end surface.
The epoxy resin which is usually employed as a bonding material for a bonded magnet can withstand a temperature of only, say, 300° C., and cannot withstand the temperature for the stress-relief annealing of a soft magnetic material which may be as high as about 1,000° C. Any attempt to heat for stress relieving a molded magnetic body made by bonding a soft magnetic material with an epoxy resin causes the epoxy resin to foam or disappear and lowers its strength and dimensional accuracy seriously. It will be possible to use water glass or a silicone resin as a bonding material having higher heat resistance than the epoxy resin, but as they have only a very low force for bonding metal particles, a large amount of bonding material is required for producing a desired bonding force. Moreover, the use of a large amount of bonding material forms an enlarged clearance among metal particles making it impossible to realize a high magnetic flux density, and resulting in a molded magnetic body of low magnetic properties.
In order to raise the mechanical strength of a compression molding made by powder molding, it is effective to raise the density of its material and it is, therefore, necessary to employ a higher molding pressure. However, if the compression molding of a material having its magnetic properties lowered by stress and strain, such as a soft magnetic material, employs an increased molding pressure to raise its soft magnetic properties, the stress and strain bearing on the soft magnetic material itself increase and thereby lower its soft magnetic properties.
While it is possible to form an irregularly shaped member by the electrical discharge machining or wire-cut electrical discharge machining of a metallic material, it is too low in mass productivity for practical use. While it is also possible to form an irregularly shaped member by a powder metallurgical method, the scattering of a bonding material during sintering or the welding of particles causes so large a dimensional change in a sintered product that a long time is required for its post treatment. Even if a product of a soft magnetic material may allow sizing for its shape correction, its processing strain lowers its soft magnetic properties seriously.